Ideal flue temperature

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Sunny Boy
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Post By: Sunny Boy » Mon. Jan. 08, 2018 2:41 pm

Scottyjr wrote:
Mon. Jan. 08, 2018 2:19 pm
No damper. I'm gonna try that. Might I be able to regulate the fire with primary dampers open to a bit above what might be used based on weather conditions, etc. and then use the MPD? Is that the idea?- Scotty

The primary damper is just one of the ways to control a fire by controlling heat volume the fire creates. But, it's not good at controlling exhaust speed through the stove, stove pipe, and chimney system.

It's like trying to control your car's speed by only using the gas pedal and never touching the brake pedal. Then, think of cold days stronger draft as driving down hill. The colder it gets the steeper the hill. Sure you can slow the car to some extent by lifting your foot off the gas pedal (closing the primary damper more), but because gasses flow faster with greater pressure differences, the hill (stronger draft) is still going to have some extra pull on that engine - which shows up on a tachometer as higher rpms.

The MPD, like a car's brakes on a hill, provides some restriction to exhaust flow in the pipe. That restriction slows down the exhaust speed so that the heat has more time to transfer through the stove and pipe walls to the room air. To get the same amount of BTU's into the room air without an MPD you'd have to burn more coal.

Another plus to using an MPD - the hottest part of the exhaust traveling through the pipe is in the center of the exhaust stream. As the hot exhaust flows around the narrowed opening of the MPD plate's edges, the pipe gets even hotter there, helping to shed a bit more heat before it reaches the chimney. You can see that affect using one of the hand-held infrared guns.

Paul

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Post By: Qtown1835 » Mon. Jan. 08, 2018 2:45 pm

I had a MK 3. Highest stack temp would read maybe 175* and stove was screaming at 550-600. MPD works great with this stove.

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Post By: rberq » Mon. Jan. 08, 2018 3:43 pm

Scottyjr wrote:
Mon. Jan. 08, 2018 11:59 am
My current stack temperature is 38% of stove temperature.
Does the ratio change significantly with the fan on vs. fan off?

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Post By: jschaefer7406 » Tue. Jan. 09, 2018 9:40 am

Paul,

We had been discussing this in a different thread, and just wanted to add to this.

My setup isn’t currently running an MPD. Have one, but haven’t installed it when replacing the flue pipe the last several times. Mine is a hand fired warm air furnace, and I do run a baro.

My initial question was regarding a draft change from .06wc to .04. I was running it at .06 for pea, and initially lowered it to .04 to burn nut.

Now, everyone says the lower draft pressure will “keep heat in the stove”. My experience is that, while the stack temp does indeed fall a bit (not much) when dropping .06 to .04, so does the plenum temp. So what I’m experiencing, is that reducing the flue draft doesn’t actually keep heat in the furnace, but only changes the range of heat. For example, I get the same heat at .06 as I did at .04, just with a different primary air setting.

So in essense, to me, changing the baro setting OR adjusting the primary air is essentially accomplishing the same thing, limiting the airflow (just on different sides of the fire).

Am I missing something?

Joe

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Post By: k-2 » Tue. Jan. 09, 2018 9:49 am

IV noticed my water temp rises when i back off on the air. Possibly not sending as much heat up the flue.

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Post By: Sunny Boy » Tue. Jan. 09, 2018 10:13 am

jschaefer7406 wrote:
Tue. Jan. 09, 2018 9:40 am
Paul,

We had been discussing this in a different thread, and just wanted to add to this.

My setup isn’t currently running an MPD. Have one, but haven’t installed it when replacing the flue pipe the last several times. Mine is a hand fired warm air furnace, and I do run a baro.

My initial question was regarding a draft change from .06wc to .04. I was running it at .06 for pea, and initially lowered it to .04 to burn nut.

Now, everyone says the lower draft pressure will “keep heat in the stove”. My experience is that, while the stack temp does indeed fall a bit (not much) when dropping .06 to .04, so does the plenum temp. So what I’m experiencing, is that reducing the flue draft doesn’t actually keep heat in the furnace, but only changes the range of heat. For example, I get the same heat at .06 as I did at .04, just with a different primary air setting.

So in essense, to me, changing the baro setting OR adjusting the primary air is essentially accomplishing the same thing, limiting the airflow (just on different sides of the fire).

Am I missing something?

Joe
Joe,
A baro controls draft pressure more by lowering the exhaust temp and not by restricting the exhaust speed like an MPD does. Both will slow the exhaust, but after the baro to the pipe thimble, the pipe temp will be lower than if an MPD is used.

As for primary control verses MPD control, or no MPD control (Scotty's situation), I did an experiment yesterday after this topic came up. Here's the numbers taken with an IR gun and a Dwyer mark II mano.

3:50 pm 32F OAT.
Primary dampers open 1/8 inch, all. MPD almost fully closed (normal settings for 32 OAT).
Top plates over firebox 680 and 715.
Pipe below MPD 129
Pipe three feet after MPD 116
Mano .01

3.55 pm MPD fully open, no other changes.

4:03 pm
Plates both 800+ (off the scale of my IR gun).
Pipe before MPD 160
Pipe 3 ft after MPD 138
Mano .055

4:05pm closed primary dampers half way to 1/16 inch all. MPD still fully open.
4:10 pm Plates 800+ but not showing dull red (means they are still below 1000F).
Pipe before MPD 170
Pipe 3 ft after MPD 143
Mano .06

Temps/mano still climbing, discontinued test and reset dampers to usual positions.

So, even though I closed the primary halfway, with affectively no MPD help, the temps and the mano readings continued to climb, verses when it had more primary opening and an almost fully closed MPD at the start.

From past experience, when I've forgotten to close the MPD, :oops: the stove temps would level off below the point that the top plates would turn dull red (1000 F), but the pipe chimney temps would continue to rise more causing even stronger draft (faster exhaust flow). And the kitchen would not get as much warmer as you might expect. However, the big sign of wasted heat is that the fuel in the firebed would only last about half as long.


Paul

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Post By: jschaefer7406 » Tue. Jan. 09, 2018 10:23 am

Thanks, Paul. Makes sense 👍🏻

When I did have the MPD installed, I can’t say I’d ever noticed a change (either in heat output, or burn times). Mostly why I left it out the last couple times. I’d reinstall it just for testing, if I could while burning 😂

I have no problem with heat output, but do wish I could extend the burn time a bit. I’m ashed up on top by morning with this new load of nut on a cold night. In my current setup (with only the baro), any suggestions?

Thanks,

Joe

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Post By: Sunny Boy » Tue. Jan. 09, 2018 11:55 am

jschaefer7406 wrote:
Tue. Jan. 09, 2018 10:23 am
Thanks, Paul. Makes sense 👍🏻

When I did have the MPD installed, I can’t say I’d ever noticed a change (either in heat output, or burn times). Mostly why I left it out the last couple times. I’d reinstall it just for testing, if I could while burning 😂

I have no problem with heat output, but do wish I could extend the burn time a bit. I’m ashed up on top by morning with this new load of nut on a cold night. In my current setup (with only the baro), any suggestions?

Thanks,

Joe
Joe,
There's a few tricks that can help extend burn times for any coal stove, but often, not by a lot.

Ash displaces fuel. Make sure you've done a thorough job of clearing ash when your getting ready to fill up the firebox for an overnight burn.

Mound the coal as high as you safely can in the firebox. More fuel = longer burns.

If you have a coal bin, dig more from the middle of the pile to get a higher concentration of small pieces of coal. Or mix in some pea coal. Either will increase the fuel density in the firebox. Plus, those smaller pieces fitting tighter together also helps slow air flow through the firebed - sorta like what an MPD would help to do.

However, if the stove is actually too small for the area it's got to heat, and you find you have to run it really hot to be comfortable, then there's no tricks to extend burn times other than get a bigger stove that can run slower and longer while still meeting heat demands.

We read complaints about coal stoves being too small to provide enough heat, ... but how often do we hear, "Help, my stove is too big" ? :D

Paul

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Post By: jschaefer7406 » Tue. Jan. 09, 2018 4:45 pm

Paul,

Thanks for the tips 👍🏻

Yeah, furnace may be undersized, hard to say for sure. On the nights of low single digits (or even negatives we just had), the house would cool from ~68 to 62 or so by morning. To me, in an 1896 house with unknown insulation, it’s acceptable. Could always be better though, I’m sure.

Seems to me that a higher CFM blower would help, as there seems to be plenty of heat in the furnace. Opened up on those nights, fan/limit dial would show ~170 degrees in the plenum. Just maybe isn’t circulating fast enough to keep up with the loss.

But yeah, not like it’s burnt out or anything, and some mornings are worse than others.

In your book, would keeping the chimney draft lower help burn times, or is it as I thought, simply reciprocal to the primary air?

Thanks,

Joe

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Post By: Lightning » Tue. Jan. 09, 2018 5:22 pm

Sunny Boy wrote:
Tue. Jan. 09, 2018 10:13 am
4:05pm closed primary dampers half way to 1/16 inch all. MPD still fully open.
4:10 pm Plates 800+ but not showing dull red (means they are still below 1000F).
Pipe before MPD 170
Pipe 3 ft after MPD 143
Mano .06

Temps/mano still climbing, discontinued test and reset dampers to usual positions.

So, even though I closed the primary halfway, with affectively no MPD help, the temps and the mano readings continued to climb, verses when it had more primary opening and an almost fully closed MPD at the start.
Holy cow Paul, I'd conclude that you have combustion air leaking into the stove somewhere with it being able to run that hot at a nearly closed primary air control.

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Post By: scalabro » Tue. Jan. 09, 2018 5:40 pm

Nice job splaining things Paul...top shelf

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Post By: Lightning » Tue. Jan. 09, 2018 6:29 pm

Yes! I do! Some how, some way it will happen! :D

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Post By: nickdearing88 » Wed. Oct. 17, 2018 5:57 pm

Wow, I re-read through this thread I started last year (and forgot about). Thanks for some good info on MPD's, and increasing efficiency. I just installed the new MPD and I want to experiment with it in various weather conditions.

I'm on top of a slight hill, and we are rather windy. And the height from boiler to chimney top is about 32'. And the chimney is interior, and has a SS liner. Thus, I've never had issues with too little draft. When burning wood, this hasn't been a problem but I want to take advantage of the lowest possible stack temps with coal.

Crazy question, does everyone open the MPD before opening the loading door, such as to just check on the fire? I can understand while shaking/loading, starting a fire, etc. but wasn't sure while "idling". I'd like to contact DS about making me a replacement door for the boiler with a glass window!

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